Showing posts with label resistance. Show all posts
Showing posts with label resistance. Show all posts

Difference between Impedance and Reactance

For all the circuits lumped elements in which time varying voltages / current exists , we find a relation in which the voltage is proportional to Current . The proportional quantity is in general a complex number and this is called Impedance.

It is a function of frequency of ‘w’ strictly speaking impedance is sum of its real and imaginary parts.

Z= R+ IX

Impedance is equivalent to a Resistance in series with pure imaginary Impedance - called a Reactance.

The voltage drop across the resistance is in phase with the current , while the voltage drop across the purely Reactive part is out of phase with the current.

The average energy loss in an impedance Z= R + iX depends only on real part of ‘Z’ and not on complex part.

There is no energy loss in Reactive part.

Checking capacitors with an ohmmeter

Set the range of Ohm meter to higher range greater than 1MOhm. The ohm-meter leads are to be connected across leads of capacitor. Fora good capacitor, the meter first shows low value and then slowly recedes to a higher value. The reading gets stabilized to  a higher value after some time due to charging action.

For ceramic, paper and Mica capacitors, the resistance falls in the range 500 - 1000 MOhm.
For electrolytic capacitors, it is around 1MOhm.

If ohmmeter reading immediately goes to zero and stays there, the capacitor is short circuited.

If capacitor shows no charging action but just reads very high resistance it may be open.

Stray capacitance:

The wiring and components in a circuit have capacitance to metal chassis. The stray capacitance is typically 5 to 10PF.  

Leakage resistance of capacitor: 

Consider a capacitor charged by a DC voltage source. If the source is removed a perfect capacitor would keep its charge indefinitely.